Conventionally, polyethylene, polypropylene, flexible polyvinyl chloride and polyethylene terephthalate and other resins have been known to have excellent flexibility, thermal resistance and water resistance. These resins have been used for garbage bags and packaging bags. However, these resins increase an amount of refuse when disposed after use and furthermore, are scarcely degradable in the natural environment. Thus, these resins semi-permanently remain underground even after burial disposal. These dumped plastics have also led to problems such as impairment of a general view and damage of life environment of marine organisms.
On the other hand, polylactic acid, a copolymer of lactic acid and other aliphatic hydroxycarboxylic acid and polyester derived from aliphatic polyhydric alcohol and aliphatic polybasic acid have been known to have thermoplastic property and biodegradability. Application of these polymers to various uses has been developed.
In these polymers, polylactic acid in particular is completely biodegraded in an animal body in a period of several months to one year. When placed in soil or sea water, these polymers initiate degradation within several weeks in a moist environment and disappear in a period of one to several years. The degradation products are characterized by lactic acid, carbon dioxide and water, which are harmless to the human body.
Further, polylactic acid is expected in recent years to extend its application field because the raw material L-lactic acid can be inexpensively produced in a large scale by a fermentation process, degradation velocity of polylactic acid is high in the compost, and polylactic acid is excellent in resistance to fungus and protection of foods from contamination of odor and color.
However, polylactic acid has high rigidity and is thus unsuited for agricultural multi-films, food packaging bags, garbage bags and other film and packaging uses which require flexibility in particular.
As an example of flexible and biodegradable resin, an inflation film prepared from polybutylene succinate has a large anisotropy in tear strength between longitudinal and transverse directions and is characterized in very weak longitudinal tear strength. Low tear strength and its anisotropy lead to problems depending upon uses. Thus, inflation films of polybutylene succinate are also unsuitable for uses in an agricultural multi-film, food packaging bag, garbage bag and other films or wrapping materials.
Generally known techniques for making resin flexible are (1) addition of plasticizer, (2) copolymerization and (3) blending of flexible polymers. However, the techniques (1) and (2) lower the glass transition temperature of the resin composition, even though sufficient flexibility can be provided. As a result, change of physical properties such as increase in crystallinity and rigidity is found under conventional environmental temperature. Additionally, use of plasticizer leads to a problem of bleeding of the plasticizer. Thus, various problems are substantially encountered for putting these techniques to practical use.
On the other hand, in the technique (3), the resin to be blended is restricted to biodegradable resin having flexibility in view of the biodegradability which is one of the subject of the invention. Such resins include, for example, polybutylene succinate, polyethylene succinate, and polycaprolactone. These resins have been disclosed in Japanese Laid-Open Patent HEI 8-245866 and HEI 9-111107. However, in order to provide sufficient flexibility (an elastic modulus of 1,000 MPa or less) for polylactic acid, the technique requires the addition of a large amount of the resin, for example, 60% by weight or more polybutylene succinate. As a result, the above characteristics of polylactic acid are impaired.
Polylactic acid has less anisotropy in tear strength as compared with polybutylene succinate. In the realities, conventional techniques cannot provide a biodegradable film having flexibility and thermal resistance without impairing characteristics of polylactic acid and further having no anisotropy in tear strength.
Consequently, the subject to be solved by the invention is to provide a biodegradable film characterized by having flexibility and thermal resistance in combination without anisotropy in tear strength.
More specifically, the object of the invention is to provide a biodegradable film having excellent thermal resistance and no anisotropy in tear strength, as a result of excellent flexibility like polypropylene, polyethylene and polyvinyl chloride which are used for garbage bags, packaging materials and outstanding resistance to blocking of film and bleeding out of plasticizer at high temperature.